More than 34.2 million US adults have diabetes, with 90-95 percent having Type 2 diabetes caused by a poor diet and lack of exercise.

Numerous studies have shown that physical activity improves heart health in Type 2 diabetic patients.

Scientists now claim that the time of day a patient exercises is critical for heart health.

“The general message for our patient population remains that you should exercise whenever you can, as regular exercise provides significant benefits for health. However, researchers studying the effects of physical activity should take into account timing as an additional consideration so that we can give better recommendations to the general public about how time of day may affect the relationship between exercise and cardiovascular health,” said study corresponding author Dr. Jingyi Qian, from Brigham and Women’s Hospital and an instructor of medicine at Harvard Medical School.

As Type 2 diabetes is more common in people who follow unhealthy lifestyles, Qian’s research focused on more than 2,000 volunteers who were also obese or overweight. They detected a link between the timing of exercise and the risk of developing heart disease.

The data used in the study came from the Look AHEAD (Action for Health in Diabetes) study, which began in 2001. It tracked the health of Type 2 diabetes patients who were overweight or obese in the United States. Participants wore accelerometer devices on their waists for a week, which measured the duration and kind of physical activity.

“The study population was very well characterized at baseline, with detailed metabolic and physical activity measurements, which was an advantage of using this dataset for our work,” said co-corresponding author Dr. Roeland Middelbeek, of the Joslin Diabetes Center in Boston.

The researchers monitored the “clock time” associated with daily moderate-to-vigorous activity. This includes physically demanding work that went beyond more conventionally recognized forms of exercise.

The researcher applied the Framingham Risk Score to determine the volunteers’ likelihood of developing coronary heart disease over the next four years. It is based on a person’s age, gender, blood pressure, cholesterol, and smoking history.

The study discovered that men who exercise in the morning have the highest risk of getting coronary heart disease, regardless of the amount and intensity of exercise they do on a weekly basis. Men who were most active during the day had lower levels of cardiorespiratory fitness. The investigators found no correlation between specific activity time and risk of coronary heart disease or cardiorespiratory fitness in women.

Sex-specific physiological variations may assist to explain the stronger connections observed in males, who are at risk at an earlier age. However, experts assert that additional factors could be at work. It is unknown why different types of movement at different times of day are related with varying degrees of health and fitness.

Additionally, the researchers were unable to account for the participants’ varied circadian rhythms. There was no way to tell whether someone was a “night owl” or a “morning lark.” While a jog at 6 p.m. may be considered “evening exercise” by one person, someone who is prone to awakening later in the day may deem it “afternoon.” This is true regardless of how the activity’s clock-time was reported in the study.

“Interest in the interaction between physical activity and the circadian system is still just emerging. We formed a methodology for quantifying and characterizing participants based on the clock-time of their physical activity, which allows researchers to carry out other studies on other cohorts,” added Dr. Qian.

Apart from advancing the integration of circadian biology and exercise physiology, the researchers are particularly excited to use longitudinal data to examine the relationship between exercise timing and cardiovascular health outcomes, particularly in diabetes patients who are more susceptible to cardiovascular events.

The results of the study were published in the journal Diabetes Care.

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